Hand Sensation
The sensory function of the human hand area is unique and essential for hand function. Protective sensibility is of fundamental importance since it protects the hand from be injured by mechanical, thermal or chemical stimuli. Functional sensibility, or tactile gnosis, helps to, without vision, define the structure of textures and to understand the shape of small items (Katz, 1989; Klatsky, et al., 1987). The sense of touch is essential for making a hand “belonging to the body”. A hand without sensory function is perceived as a foreign body and may even be denied by the owner (Ramachandran, 1998). In addition, regulation of grip force and execution of delicate motor task in the hand are dependent on sensory input from the hand to the central nervous system.
The sensibility of the glabrous skin of the hand is based on four types of mechanoreceptors, localised in subepidermal and subcutaneous areas, responding to static pressure of vibrotactile stimuli (Johansson, Birznieks, 2004; Johansson, Vallbo, 1979). Among receptors responding to vibration are Meissner's corpuscles, located in the subepidermal papillae, with small receptive fields (fast adapting type I-FAI receptor) and Pacini's corpuscles, located in subcutaneous layers possessing large receptive fields (fast adapting type II-FAII receptor). The Merkel cells, located just beneath epithelium, respond to static pressure and have small receptive fields (slowly adapting-type I-SAI receptor). Ruffini's organ, located subcutaneously, responds mainly to stretching (SAII receptors).
The Cortical Body Map
The various parts of the body are represented in projectional areas in the sensory and motor cortex of the brain, constituting a cortical body map (Kaas, 1997; Merzenich, Jenkins, 1993). In somatosensory cortex the projectional area of the various body parts are in proportions to their sensory functions: body parts with exceptionally well developed sensation like the hand or face occupy a major part of sensory brain cortex.
Electrical signals, elicited by touching the hand are transferred via nervous pathways primarily to contralateral sensory brain cortex, here constituting a neural map of the hand, also called the cortical hand map. In primates, exact hand- and finger representations have been meticulously outlined by direct recording from the cortical surface (Kaas, 1997; Merzenich, Jenkins, 1993; Merzenich, et al., 1978; Merzenich, et al., 1987), and in humans a corresponding cortical mapping of the hand has been identified by use of various brain imaging techniques such as magneto-encephalography (MEG) and functional magnetic resonance imaging (fMRI) (Hari, et al., 1993; Naas, 1997; Maldjian, et al., 1999; van Westen, et al., 2004). In the cortical hand map the individual fingers are well separated by sharp boarders, the thumb being located inferiorly in relation to the fifth finger. The forearm projectional area is located immediately superior to the little finger.
Brain Plasticity and Cortical Competition
It was long believed that the cortical body map was firmly established in the adult brain, that the brain was “hard-wired” from the start and that sensory body representations in the mature brain was fixed and not capable of functional reorganisations. However, according to evolving concepts over the past decades, the brain is much more plastic than was previously believed possessing a very substantial capacity for cortical functional reorganisations even at the adult stage (Bach-y-Rita, 1967; Bach-v-Rita, 1981; Bach-y-Rita, 1990; Bach-y-Rita, 1994; Buonomano, Merzenich, 1998). In adult primates there is a capacity for rapid cortical reorganisations in the sensory cortex (Merzenich, et al., 1978; Merzenich, et al., 1983; Merzenich, et al., 1987; Merzenich, et al., 1984). The cortical projection of the hand is experience-dependent and depending on factors like activity and extent of sensory inflow. For instance, amputation of an arm results in total arrest of all sensory inflow from the arm to the brain—a so called de-afferentiation. In such cases there is a rapid displacement of the adjacent face area towards the hand representation in sensory cortex (Elbert, et al., 1994), which may give rise to a strange clinical phenomenon already 24 hours after an arm amputation: the missing hand can be mapped in the face so that touch of specific areas of the face can give rise to tactile sensations in individual fingers of the missing hand (Borsook, et al., 1998; Flor, et al., 1998; Flor, et al., 1995; Ramachandran, et al., 1992).
Central Nervous Effects of Cutaneous Anaesthesia Fast functional changes in cortical representation may be induced also as a result of anaesthetic blocks. Finger anaesthesia of healthy voluntaries results, within minutes, in a cortical expansion of the adjacent fingers which hereby occupy areas that cover the former projection site of the anaesthetised finger (Rossini, et al., 1994). Cutaneous anaesthesia of the forearm, using prilocaine/lidocaine (EMLA) results in rapid improvement of sensory functions in the hand, presumely due to expansion of the cortical hand sensory projection giving the hand access to more brain space (Bjorkman, et al., 2004), and in nerve injured patients repeated application of prilocaine/lidocaine to the forearm results in enhanced sensory recovery of the hand (Rosen, et al., 2006). Preliminary studies indicate that the principle is valid also for the lower extremity: application of prilocaine/lidocaine to the calf of the lower limb results in improved sensation in the sole of the foot in healthy voluntaries.
Example 1. A 62 year old female dental technician, working with vibrating tools for 15 years, suffer from neuropathy with impaired sensibility of the hand. She experienced numbness and impaired fine discriminative sensibility of the hand. 40 g of EMLA crème was applied to the volar aspect of the forearm within a 5×15 cm cutaneous area. The crème was covered with a thin plastic membrane, in turn covered with a piece of textile (a thin towel) designed to fit the size and shape of the forearm, wrapped around the forearm and fixated to itself by tape. The wrapping was applied for one hour. Before and one hour after application of EMLA crème two-point discrimination, perception to touch and vibration thresholds were assessed in the long finger. After EMLA application two-point discrimination improved from 4 to 2.2 mm, and capacity to feel touch improved from 0.5 g to 0.04 g. Repeated treatment sessions for two mounts resulted in persistent improvement, and in a VAS-scale (1=worst possible, 10=best possible) the score improved from 2 to 8.
Example 2. In a healthy 26 year old volunteer the sensibility of the sole of the foot was assessed with focus on vibration sense volar to the first metatarsal head. 40 g EMLA crème was applied to the calf over a skin area distal to the knee measuring 10-15 cm. In analogy with example 1 the EMLA crème was covered with a thin plastic membrane, in turn covered with a piece of textile, designed in shape after the size and shape of the calf, wrapped around the lower leg below the knee and fixated to itself by tapes. EMLA crème was applied for one hour and then removed. The treatment resulted in significantly improved vibration sense with thresholds for vibration perception decreasing from 130 to 120 dB whitn 250 Hz, and from 115 to 105 dB within 15 Hz.